Validating XML with XSDs ... but still allow extensibility

Maybe it's me, but it appears that if you have an XSD

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="User">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="GivenName" />
                <xs:element name="SurName" />
            </xs:sequence>
            <xs:attribute name="ID" type="xs:unsignedByte" use="required" />
        </xs:complexType>
    </xs:element>
</xs:schema>

that defines the schema for this document

<?xml version="1.0" encoding="utf-8" ?>
<User ID="1">
    <GivenName></GivenName>
    <SurName></SurName>
</User>

It would fail to validate if you added another element, say EmailAddress, and mix up the order

<?xml version="1.0" encoding="utf-8" ?>
<User ID="1">
    <SurName></SurName>
    <EmailAddress></EmailAddress>
    <GivenName></GivenName>
</User>

I don't want to add EmailAddress to the document and have it be marked optional.

I just want an XSD that validates the bare minimum requirements that the document must meet.

Is there a way to do this?

EDIT:

marc_s pointed out below that you can use xs:any inside of xs:sequence to allow more elements, unfortunately, you have to maintain the order of elements.

Alternatively, I can use xs:all which doesn't enforce the order of elements, but alas, doesn't allow me to place xs:any inside of it.